1. Field of the Invention
The present invention relates to a lighting unit and a liquid crystal display device using the lighting unit, and more particularly relates to a structure of a light guide plate used in the lighting unit.
2. Description of Related Art
Liquid crystal display (LCD) devices have several features, such as thinness, lightweight and low power consumption. Because of these features, LCD devices are widely used in fields including Office Automation (OA) machines, Audio and Visual (AV) appliances and cell phone terminal devices. In case of transmissive LCD devices, the display includes a backlight to irradiate a rear surface of the LCD panel. The LCD device indicates information by controlling the amount of transmitting rays of light emitted from the backlight light sources. Fluorescent lamps have been used as the backlight light sources. In recent years, however, light-emitting diodes (LEDs) are used instead of the fluorescent lamps. That is because fluorescent lamps need peripheral circuits, such as inverter circuits, to turn on the lamps, and because the inverter circuits for the fluorescent lamps make noise. Especially, LEDs emitting red (R) green (G) blue (B) color, respectively, are used together to obtain a white color for backlighting by mixing the three colors.
A LCD device of a conventional type using LEDs for R, G and B colors as the light sources is made up of LED light sources, a LCD device, one or more light guide plates, a reflection plate, and an optical sheet. The LED light sources emit rays of light of R, G and B colors. The light guide plates guide emitted light from the LED light sources to the LCD panel. The reflection plate reduces loss of light transmitted to the LCD panel. The optical sheet distributes light supplied to the LCD panel uniformly.
Each of the LEDs serves as a point light source. Each of the LEDs is usually provided with a lens in order to provide directivity of light intensity as regards an angle of external light emissions. The directivity of light intensity causes the LED chip to emit rays of light with a brightness corresponding to the angle. For this reason, in the vicinity of each of the LEDs, rays of light tend to influence the directional characteristics of the LEDs. As a result, the arrangement of LEDs causes high and low intensities of the ray of light. In particular, this causes a problem in that the high and low intensities of the rays of light are displayed on the LCD panel in the form of color irregularity in a case where the white color is realized by use of LEDs respectively for the R, G and B colors.
With this taken into consideration, the dimension of the light guide plate from the LED light sources to the LCD panel is elongated to reduce color un-uniformity caused by color mixing of different color of light. A group of point light sources is arranged away from the edge of the light guide plate in order that the intervals of the adjacent LEDs and the distance between the group of LEDs and the edge of the light guide plate can satisfy predetermined relationships. This method is proposed in Japanese Patent Laid-open No. 2000-36209.
In a case where a plurality of types of LEDs emitting mutually different colors are intended to be used as light sources, it is important that rays of light respectively representing mutually different color rays of light should be evenly mixed with one another in the aforementioned manner to obtain a uniform mixed color. For this reason, the dimension of the light guide plate from the LED light sources to the LCD panel is designed to be elongated. However, this method imposes restrictions on the structural design because a long distance is required for color mixing from the LED light sources to a display area. As a result, the dimension of the light guiding plate needs to be enlarged, and the size of the LCD device as a whole is accordingly enlarged. In addition, the weight of it is increased.